1. Field of the Invention
This invention relates to a focus detecting device for a camera having both passive system focus detection means which utilizes ambient light for focus detection (hereinafter referred to as "passive system") and active system focus detection means which utilizes for focus detection artificial light emitted by a light projecting element and reflected from an object to be photographed (hereinafter referred to as "active system").
2. Description of the Prior Art
In interchangeable lens type cameras, it is a general trend to employ a T.T.L. (through the lens) focus detecting device a T.T.L. focus detecting device so called a passive system which involves no limitation in object distance, has been most frequently employed in order to cope with the use of a variety of interchangeable lenses having different focal lengths and different F values at open aperture, but this system has a difficulty in that if the ambient light is not bright enough, good accuracy in focus detection is not obtainable or focus detection is not made possible. In order to overcome this difficulty, a complementary technique, so called an active system, has been employed in which, when the ambient light is dim, the object is illuminated through positive emission of an artificial light so as to help the focus detection. In this case, however, there is a problem that a near infrared beam which is not or little sensitive to the human eye is used for such illumination, the focus position deviates from that in the case of a visible light ray being used as a result of a variation in chromatic aberration for each interchangeable photographing lens between the near infrared beam and the visible light.
In an attempt to solve this problem, a variety of techniques have been proposed in the art.
(I) One prior art disclosure for such purpose is found in Japanese Published Unexamined Patent Application No. 57-154224. The camera disclosed therein has a focus detecting sensor for a focus condition detection on the basis of a light beam transmitted through an interchangeable photographing lens mounted on a camera body. For such a sensor, one having a wide wavelength sensitivity range (visible light.about.infrared rays) and high sensitivity to wave lengths of the infrared region is employed. Each interchangeable photographing lens used for the camera is provided with a mechanical signaling member which signals a focus deviation value corresponding to a difference in focal point between visible light and infrared radiation, and the camera body is provided with a shift member for shifting the sensor according to the signal from the signaling member so that in-focus photographing may be performed under visible light even when photographing lenses having different infrared chromatic aberrations are used.
(II) Another type of prior art disclosure is found in Japanese Published Unexamined Patent Application Nos. 58-59413 and 58-86504. The camera disclosed therein has a color temperature measuring means and an adjusting means for shifting each photographing lens according to the output of the color temperature measuring means so that a focus deviation may be corrected to a chromatic aberration value corresponding to the relevant one of the focal point differences between visible light and infrared rays which are stored in the lens.
(III) A further prior art disclosure is found in Japanese Published Unexamined Patent Application No. 59-129839. The camera disclosed therein has a focus detection sensor which is sensitive only to visible light and a focus detection sensor which is sensitive only to infrared rays. Each photographing lens has a read only memory (ROM) in which a focus deviation value corresponding to a focal point difference between visible light and infrared radiation is stored, and a discrimination element for determining which sensor is to be used for focal point detection according to the difference in the quantity of light between visible light and infrared radiation, whereby if the infrared ray sensor is used for focus detection, the focus detection signal obtained based on a signal from the sensor is corrected by a signal from the ROM corresponding to the focal point difference between the visible light and the infrared ray so that a corrected focus detection signal is issued on the basis of which focus adjustment is effected for the photographing lens.
Now, with the prior art arrangement described in (I) above, the difficulty is that since a sensor having a wide wavelength sensitivity range is employed as a focus detection sensor, a variation will be produced in chromatic aberration value according to the difference in color temperature between the two different light sources, i.e., ambient light and infrared rays utilized in illuminating the object, the variation in chromatic aberration value resulting in a variation in the focus position. This means that a mere correction according to a difference in focal point between visible light and infrared light is not sufficient to permit accurate focus detection.
The prior art arrangement described in (II) above has no such drawback with pointed out as regards to the first mentioned arrangement, but it has a disadvantage in that a color temperature measuring means is necessarily required, the entire setup is necessarily complicated with respect to the relative disposition of the optical system for color temperature measurement and the optical system for focus detection.
With the prior art arrangement described in (III) above, the difficulty is that two sensors, one for visible light and the other for infrared light, being necessarily required, the optical system is complicated, with a complex process of electrical processing involved.